Crop losses due to plant parasitic nematodes are estimated to exceed $100 billion. Preventing this damage represents a significant challenge. With the impending loss of the fumigant methyl bromide, there is insufficient time to develop and register new synthetic compounds for nematode control. Therefore, other options are needed.
Phytopathogenic nematodes are particularly difficult to control because they are covered with a thick, impermeable cuticle, or outer covering, and have very few sensory neurons. Since many pest control compounds operate as neurotoxins, the low number of neurons exposed by phytopathogenic nematodes decreases the effective target area for nematicidal compounds and has resulted in the development of nematicidal compounds with extremely high neurotoxic properties.
Furthermore, because phytopathogenic nematodes are found in soil or plant roots, exposing the phytopathogenic nematodes to control agents is difficult to achieve and puts the water table at risk of contamination from these toxic compounds. The use of nematicides based on neurotoxins has been demonstrated to contaminate both ground and surface water. Consequently, many of these compounds are being removed from the market for public health reasons.
Fumigation of soil prior to planting is a popular method for controlling nematodes. One of the most popular fumigants, methyl bromide, is slated for removal from use because of its ozone destroying properties. Furthermore, this practice of soil fumigation kills organisms in soil indiscriminately and runs the risk of eliminating beneficial microbes as well as disease-causing organisms. Therefore, an effective nematicide with benign environmental effects is urgently needed.
Pasteuria was first described in 1888 by Metchnikoff (Annales de I'Institut Pasteur 2:165-170) as a parasite of water fleas. Subsequently, Cobb described a Pasteuria infection of the nematode Dorylaimus bulbiferous (2nd ed. Hawaiian Sugar Planters Assoc., Expt. Sta. Div. Path. Physiol. Bull. 5:163-195, 1906).
The life cycle of the bacteria involves a stage when endospores bind to the cuticle of the nematodes in soil. P. penetrans then proliferates within the nematode body and passes through several morphological phases, including mycelial structures and thalli, culminating in the development of new endospores. Endospores are released when the nematode body lyses.
Growth of the bacteria within the nematode body reduces or eliminates the production of eggs by the nematode, severely restricting the rate of nematode reproduction. Economic damage to the host crop normally is inflicted by the first generation progeny of nematodes and is prevented by Pasteuria through lowering the concentration of progeny nematodes in the plant root zone.
Although the use of Pasteuria to control nematodes has been previously proposed, a number of factors, including sub-optimal delivery options have limited the use of this nematode control strategy. Conventional methods for controlling nematodes using Pasteuria strains include applying the bacteria to the plant and soil in free form (e.g. Stirling G. R. 1984. “Biological control of Meloidogyne javanica with Bacillus penetrans”, Phytopathology, 74:55-60) or in solid and liquid formulation (e.g. U.S. Pat. No. 5,248,500). However, despite the highly selective effectiveness of the bacteria on the phytopathogenic nematodes, they need to be in contact with the nematodes in order to produce the nematocidal effect. When applied directly to the soil, a large amount of the bacteria is required and they have to be mixed well with the soil, which substantially increases the cost of using the bacteria.
While various biocontrol methods using Pasteuria bacteria are known, there still remains a need for an improved approach for using these bacteria to effectively control nematodes. Therefore, the subject invention provides novel methods for controlling phytopathogenic nematodes that attack plants.